Most of my writing is academic and involves lots of footnotes. Sometimes it
is useful to be able to combine two separate files, each of which have
footnotes. How do I do so in a way that automatically renumbers the
footnotes in the file that comes second?

More specifically, I have file1.org and file2.org, and each of them have
three footnotes ([fn:1], [fn:2], and [fn:3]). I want to create file3.org,
which simply merges file1.org and file2.org, with the contents of file2.org
coming after the contents of file1.org. The problem is that the footnote
calls ([fn:#]) in the file2 section will end up pointing the file1
footnotes. What I really want is to automatically renumber all the
footnotes [fn:1]...[fn:6], but I don't know how to do this, given that org
permits multiple footnote calls to the same footnote.

Thanks,
Alan